代码实现：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

void convertBiNode_pro(struct TreeNode *root, struct TreeNode **p) {
    if (root) {
        convertBiNode_pro(root->left, p);
        (*p)->right = root;
        (*p) = (*p)->right;
        (*p)->left = NULL;
        convertBiNode_pro(root->right, p);
    }
}

struct TreeNode* convertBiNode(struct TreeNode *root) {
    if (root == NULL) {
        return root;
    }
    struct TreeNode *h = root;
    while (h->left) { // 找到链表的头
        h = h->left;
    }
    struct TreeNode **p = malloc(sizeof(struct TreeNode*));
    *p = malloc(sizeof(struct TreeNode));
    convertBiNode_pro(root, p); // 递归，中序遍历
    return h;
}